Cobalt-base alloys

ABSTRACT

ALLOYS CONSISTING OF AT LEAST 45% COBALT AND PARTICULAR AMOUNTS OF CARBON, CHROMINUM, TUNGSTEN AND ALUMINUM, HAVE A HIGH RESISTANCE TO OXIDATION AND HOT-CORROSION ATTACK AND, ALSO, A HIGH MECHANICAL STRENGTH AT TEMPERATURES UP TO ABOUT 900* C. THE ALLOYS MAY ALSO CONTAIN IRON AND/OR NICKEL.

Feb 27, 1973 M. A. P. DEWEY 3,718,461

COBALT-BASE ALLOYS Filed Aug. 19, 1971 STELL/TE 31 TEST CONDITIONS {32 SAL T U) 5 0- 450 5 Q 2 E g J L) %4-5- Q (Li a 400 2 E i l 2 S3 0- g k I E E u -50 E (L ALLOY 0 TIME IN HOURS United States Patent U.S. Cl. 75--171 16 Claims ABSTRACT OF THE DISCLOSURE Alloys consisting of at least 45% cobalt and particular amounts of carbon, chromium, tungsten and aluminium, have a high resistance to oxidation and hot-corrosion attack and, also, a high mechanical strength at temperatures up to about 900 C. The alloys may also contain iron and/or nickel.

This invention is concerned with cobalt-base alloys which have excellent resistance to oxidation and hot-corrosion attack, combined with high mechanical strength up to temperatures of 900 C.

Alloys based on a cobalt or nickel matrix are known; certain of them are known as superalloys because of their use for components operating at a high temperature, such as in gas turbine engines. Such cobalt or nickel matrix alloys are generally of two types:

(1) Alloys in which chromium is added to the cobalt or nickel to provide resistance to oxidation and hot-corrosion attack, the alloys containing chromium in an amount of from 20 to 30%. Other alloying additions, in particular tungsten, manganese, silicon, molybdenum, iron and carbon, may be made for matrix strengthening and improved high temperature properties. These alloys, of which the cobalt-base Stellite 31 (trademark) is a typical example, have high resistance to oxidation and hot-corrosion, but only moderate strength at room and elevated temperatures.

(2) Alloys in which the addition of chromium to the cobalt or nickel is limited to not more than approximately 20%, but which contain additions of one or more of the elements aluminium, titanium, tantalum and/or niobium in amounts up to approximately 6% each, to strengthen the alloy by precipitation of intermetallic compounds containing these elements. These alloys, of which .the nickel-base Nimonic 90 (trademark) is a typical example, combine moderate resistance to oxidation and hot-corrosion with high strength at room and elevated temperatures.

Hitherto, cobalt or nickel-base alloys have not been available which combine the optimum properties of these two types of alloy, that is which combine high strength at room and elevated temperature with high resistance to oxidation and to hot-corrosion, for example by halides. A range of cobalt-base alloys has now been developed, which alloys have this advantageous combination of properties.

The new cobalt-base alloys of the invention consist, by weight, of:

carbon: 0.01 to 0.10% chromium: 15.0 to 35.0%preferably from 18.0 to

22.0% or from 28.0 to 32.0%

3,718,461 Patented Feb. 27, 1973 "ice tungsten: 5.0 to 15.0%-preferably from 5.0 to 10.0%

aluminium: 5.0 to l2.0%-preferably from 6.0 to 12.0%,

particularly from 6.5 to 8.0%

manganese: 0.5 to 2.0%

the balance being cobalt, which constitutes at least 45% of the alloy, and unavoidable impurities.

The alloys may additionally contain up to 15% of iron and/or up to 15% of nickel.

It is believed that the advantageous properties of these alloys is due to the presence of aluminium which serves to strengthen the matrix by the precipitation of intermetallic compounds and also imparts high resistance to oxidation and hot corrosion. Aluminium also imparts hardness to these alloys and both aluminium and tungsten impart hardness. This is due to a microstructure consisting of an intimate mixture of grains and particles of cobalt, also containing in solid solution the elements chromium, tungsten, manganese, aluminium and, if present in the alloycomposition, iron and nickel, and grains and particles of the ordered body centred cubic compound CoAl, which can also contain in solid solution the elements chromium, tungsten, manganese, iron and nickel.

In order that the invention may be more fully understood, the following examples are given by way of illustration only:

EXAMPLES Alloys of the following compositions A to F were prepared, in each case the balance of the alloy composition consisted of cobalt and unavoidable impurities. 'It should be noted that alloy E falls outside the scope of this invention and does not show the properties described, because of the absence of tungsten. (Aluminium contents less than 5% have a similar detrimental effect.)

Alloy A: Percent Carbon 0.01 Chromium 20.1 Tungsten 7.5 Iron 8.2 Nickel F- 10.3 Manganese 0.5 Aluminium 6.0

Alloy B:

Carbon 0.02 Chromium 20.2 Tungsten 7.3 Iron 7.9 Nickel 9.9 Manganese 0.45 Aluminium 7.1

Alloy C:

Carbon 0.02 Chromium 19.9 Tungsten 7.45 Manganese 1.4 Aluminium 7.0

Alloy D:

Carbon 0.02 Chromium 20.0 Tungsten 7.4 Manganese 1.45 Aluminium 7.9

3 Alloy E:

Carbon 0.04

Chromium 19.7 Iron 8.1

Nickel 9.8

Manganese 1.5 Aluminium 5.4 Alloy F:

Carbon 0.04

Chromium 19.7

Tungsten 7.45 Iron 8.3

Nickel 10.1

Manganese 1.4 Aluminium 6.6

Ingots were prepared of these alloys in a conventional manner which may be summarised as follows: melting and alloying were carried out in an induction heated vacuum furnace. The melt temperature was maintained at 1550 C. and the alloys were cast into ingot moulds made of steel to form ingots 2" in diameter. These ingots were then extruded at 1300 C. to rod 0.5" in diameter. Samples of the rod were heat treated at a temperature of 1250 C. followed by air cooling to room temperature, re-heated to a temperature between 700 and 980 C. and aged for times of 1 to 90 hours. These alloys were tested for resistance to oxidation at temperatures up to at least 1000 C. in comparison with the commercially available alloys, Stellite 31 and Nimonic 90, both of which are widely used because of their good high temperature oxidation resistance. The nominal compositions of the alloys Stellite 31 and Nimonic 90 are:

Stellite 31: Percent Carbon 0.45-0.60 Chromium 23.0-28.0

Nickel 9.012.0 Tungsten 6.0-9.0 Iron, max. 2.00 Cobalt Balance Nimonic 90:

Carbon, max. 0.13 Silicon, max 1.5 Iron, max 3.0 Manganese, max. 1.0 Chromium 18.0-21.0 Titanium 1.8-3.0

Aluminium 0.8-2.0 Cobalt 15.0-21.0 Nickel Balance The high temperature oxidation test consisted of heating cylindrical samples of known weight in air at 1000 C. for 25 hours, after which they were removed from the furnace and allowed to cool to room temperature. The oxide film formed on each sample was examined for evidence of spalling and the weight of each sample re-determined. This sequence was repeated four times, when the test was stopped, at which point the sample had been oxidised for a total time of 100 hours. The samples were then sectioned and metallographically examined and the depth of internal oxidation measured. The results were:

TABLE I Weight Internal oxidation change,

mgJcmfi/hr. Depth,;im Type 8 Uniform preferential +6014 50 oxidation +0:0]5 35 0t CoAl particles.

}Intergranular.

In addition to their much better resistance to oxidation, as show y e ower we ght ch g a es in Table I.

the alloys according to the present invention also have the advantage that oxidation occurs uniformly without preference for the grain boundaries. In the Stellite 31 and Nimonic 90, alloys, preferential intergranular oxidation occurs and this is particularly detrimental to the mechanical properties after oxidation.

Tests for assessing resistance to the form of high temperature corrosion known as sulphidation, which involves attack of the metal by a combination of chloride and sulphate ions, were also carried out on one alloy, D, in comparison with the Stellite 31. This latter alloy is generally considered to have the best sulphidation corrosion resistance of all the alloys which are currently commercially available. These tests were carried out in a rig specially designed to simulate the operating conditions in a gas turbine engine. Cylindrical samples of known weight were spun during the test, which was conducted at a temperature of 870 C. with 4 parts of synthetic sea salt DEF 1053 per million parts of air injected into the burner. Six specimens of each alloy were tested and removed at successive 20 hours intervals, metallographically sectioned and the depth of corrosion attack measured. The results are presented in graphical form in the single figure of the accompanying drawing in which the depth of penetration of the surface is plotted against time. The curves show that alloy D has much better resistance to this form of corrosion than Stellite 31.

The tensile properties of alloys A to D were also determined at room and elevated temperatures and are summarised in Tables II and III in comparison with those of Stellite 31 and Nimonic 90.

TABLE II 0.1% proof stress, UTS, Elong., Ageing treatment tont./in. 2 tout/111. percent Alloy:

A 1h0ur,900 C 52.0 76.0 8 B do 55.0 70.0 8 C.. o 67.0 80.0 13 C 20110urs, 800 C 75.0 86.0 4 D 8 hours, 080 C 70. 0 88. 0 5. 5 Stellite 31 As cast. 33.0 50. 5 8 Do 50 hours, 737 C 40.0 57.0 2 Nimonic 16 hours, 700 C 51.0 81.0 27

TABLE III Test 0.1 71 temperproof UTS,

Ageing trcatature, Stress, t0nf./ Elong., ment tonf./in. in. percent Alloy:

A 1 hour, 000C-- 800 20. 5 24. 0 37 B d0 800 24. 0 2G. 0 25 C .do 800 2A. 0 28. 0 55 C 20 hours, 800 C. 800 31.0 33.0 10 C.. 0 750 42.5 48.0 21 C 0 700 50.0 03.0 14 8 hours, 080 C- 800 31.0 33. 0 42 As cast 818 27. 0 16 0 50 hours, 737 C- 818 18. 5 27. 0 10 Nimonic 00... 16 hours, 700C 700 44. 0 63. 0 10 Do .do 800 35. 0 45. 0 10 The hardening effect of the combination of Al and W is shown in Table IV.

TABLE IV Max. hardness VPN, aged 900 C. Alloy E 305 Alloy F 455 The above results show that the alloys according to the invention possess mechanical properties of the same order as those of Nimonic 90 and superior to those of Stellite 31, while having oxidation resistance superior to that of both Nimonic 90 and Stellite 31 and corrosion resistance superior to that of Stellite 31.

I claim:

1. A cobalt-base alloy which has a high resistance to oxidation and hot corrosion attack and has a high mechanical strength at temperatures up to 900 C., which alloy consists of, by weight:

Percent Carbon 0.01-0.10 Chromium 15.0-35.0 Tungsten 5.0-15.0 Aluminium 5.0-12.0 Manganese 0.05-2.0

the balance being cobalt, which constitutes at least 45% by weight of the alloy, and unavoidable impurities.

2 An alloy according to claim 1 which contains from 18.0 to 22.0% chromium.

3. An alloy according to claim 11 which contains from 28.0 to 32.0% chromium.

4. An alloy according to claim 11 which contains from 5.0 to 10.0% tungsten.

5. An alloy according to claim 1 which contains from 6.5 to 8.0% aluminium.

6. A cobalt-base alloy which has a high resistance to oxidation and hot-corrosion attack and has a high mechanical strength at temperatures up to 900 C., which alloy consists of, by weight:

Percent Carbon 0.01-0.10 Chromium 15.0-35.0 Tungsten 5.0-15.0 Aluminium 5.0-12.0 Manganese 0.05-2.0

and, optionally, up to 15% of iron and, optionally, up to 15 of nickel, the balance being cobalt, which constitutes at least 45% by weight of the alloy, and unavoidable impurities.

7. An alloy according to claim 6 which contains from 18.0 to 22.0% chromium.

8. An alloy according to claim 6 which contains from 28.0 to 32.0% chromium.

9. An alloy according to claim 6 which contains from 5.0 to 10.0% tungsten.

10. An alloy according to claim 6 which contains from 6.5 to 8.0% aluminium.

11. A cobalt-base alloy which has a high resistance to oxidation and hot-corrosion attack and has a high mechanical strength at temperatures up to 900 C., which alloy consists of, by weight:

the balance being cobalt, which constitutes at least 45% by weight of the alloy, and unavoidable impurities.

112. A cobalt-base alloy according to claim 11 which consists substantially of:

Percent Carbon 0.01 Chromium 20.1 Tungsten 7.5 Iron 8.2 Nickel 10.3 Manganese 0.5 Aluminium 6.0

balance cobalt and unavoidable impurities.

13. A cobalt-base alloy according to claim 11 which consists substantially of:

Percent Carbon 0.02 Chromium 20.2 Tungsten 7.3 Iron 7.9 Nickel 9.9 Manganese 0.45 Aluminium 7. 1

balance cobalt and unavoidable impurities.

14. A cobalt-base alloy according to claim 11 which consists substantially of:

Percent Carbon 0.02 Chromium 19.9 Tungsten 7.45 Manganese 1.4 Aluminium 7.0

balance cobalt and unavoidable impurities.

15. A cobalt-base alloy according to claim 11 which consists essentially of:

Percent Carbon 0.02 Chromium 20.0 Tungsten 7.4 Manganese 1.45 Aluminium 7.9

balance cobalt and unavoidable impurities.

16. A cobalt-base alloy according to claim 11 which consists essentially of:

RICHARD O. DEAN, Primary Examiner US. Cl. X.R. 14832.5, 158 

